This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. To improve the efficiency of an E. coli Biotin Ligase (BirA) mediated protein tagging method, efforts are being made to increase the promiscuity of BirA by protein engineering of its binding pocket. Our contribution toward these engineering efforts uses X-ray crystallography to study the contacts between BirA and its substrates.